The present invention relates to image processing apparatus, and more particularly to an image processing apparatus having a digital image processing section including enhancement of edges in image to be displayed. When an image is processed for edge enhancement, a technique is generally used to extract edge components within the image so as to add the extracted edge components to the original image. In such case, the edge components to be added are obtained by extracting high-frequency components of luminance signals of the image. In image pickup devices using RGB primary-color filter, the techniques for generating luminance signal to extract such high-frequency components include: (1) technique for generating luminance signal by using only G (green) signals out of all the color signals of RGB within the image; and (2) technique for generating luminance signal by using all the color signals of RGB in the image.
An example of construction of conventional image processing apparatus will now be explained by way of FIG. 1 in which such two techniques are used to generate luminance signal to perform edge enhancement processing. Referring to FIG. 1, numeral 101 denotes an image pickup means; image signals of Bayer RGB array are obtained from the image pickup means 101. The Bayer RGB array image signals are subjected to white balancing at a white balancing section 102. Luminance data are then generated from 2G or (R+2G+B)/2 at a luminance data generating section 103 where R, G, B represent the outputs of the color signals of R (red), G (green), B (blue), respectively. The luminance data are then passed through a high-pass filter 104 to extract edge components and the degree of edge enhancement is adjusted at an edge enhancement adjusting section 105 to form edge signals.
On the other hand, image signals after the white balancing are separated of colors by pixel as a unit into RGB signals at color separation circuit 106 and then subjected to color correction and γ correction at a color correction/γ correction circuit 107. The RGB signals are converted into Y, Cr, Cb signals at YC conversion section 108. The above described edge signals are then added to the luminance signal Y outputted from YC conversion section 108 at an adder circuit 109 to obtain edge-enhanced luminance signal Y.
The above two techniques have their respective merits and demerits. In particular, FIG. 2A shows a part of image consisting of pixels of Bayer RGB array. In such image of Bayer RGB array, two times of image signals S are used in the case of generating luminance signal Y from 4 pixels of RGB (R+2G+B) as shown in FIG. 2B as compared to the case of generating luminance signal Y as shown in FIG. 2C from two pixels of G. The resulting noise N, on the other hand, is only 21/2 times from the theory of noise reducer. For this reason, supposing S/N at the time of generating from two pixels of G as 1/1=1, that at the time of generating from 4 pixels of RGB becomes 2/21/2=21/2. The case of generating from 4 pixels of RGB becomes 21/2 times better. Since, however, the signals of R, B, which contain not much genuine luminance information, are used as luminance signal when 4 pixels of RGB are used, excessively emphasized edges are generated and defects occur especially at the portion of boundary between two highly saturated colors.
As has been described, there are merits and demerits in both the case of using only G signal and the case of using all the color signals of RGB in generating luminance signal. These are summarized in Table 1.
TABLE 1Luminance signalEdge noise of highEdge noise of lowgeneration methodS/Nsaturation portionsaturation portionFrom only GBadSmallLargeFrom RGBGoodLargeSmall
Prior-art techniques according to some specific documents will now be described. Japanese patent application laid open Hei-10-108208 for example discloses a technique in which contour signals (edge components) are extracted from G signals to perform edge enhancement. Further, Japanese patent laid open application Hei-9-261513 discloses a contour (edge) enhancing method in which two types of contour extracting methods, i.e., MIX (mix) mode and NAM (non-average mix) mode are switched to each other according to the saturation of image. Here, MIX mode refers to generation of contour enhancement signal after addition of a plurality of signals corresponding to the three primary colors of RGB, etc., in the light of image to be displayed; NAM mode refers to generation of contour enhancement signal by adding one selecting the signal emphasized most in the direction of black and one selecting the signal emphasized most in the direction of white out of the respective contour enhancement signals of the above described plurality of signals.
Thus the problem is that there are respective merits and demerits as described above of the technique for generating luminance signal by using only G signals in the image to extract edge components and the technique for generating luminance signal by using all the color signals of RGB in the image to extract edge components. Further, the contour enhancement technique as disclosed in the above Japanese patent laid open application Hei-9-261513 has a problem that, if Bayer RGB array is used as the color filter array of image pickup device, the mere switching between the two types of contour extracting methods according to saturation alone may, depending on the colors of image, fall short of suitable processing due to lack of information. Furthermore, there is a problem of complicated construction in the NAM mode, since it is necessary to perform addition by respectively selecting one emphasized most in the direction of black and one emphasized most in the direction white out of the contour enhancement signals.